The present specification generally relates to high-gain amplifiers and particularly to pixel sensor readout amplifiers.
Image sensors, such as active pixel sensors described in U.S. Pat. No. 5,841,126, require that the number of photons that are received be accurately counted. Accurate counting often requires small level signals to be amplified. The amplifiers in certain cases are charge amplifiers.
A charge amplifier can be conceptually thought of as an operational amplifier "op-amp" with capacitors at the input and the feedback between the output and the input. The gain is proportional to the ratio of the values of these capacitors. However, making a larger capacitor requires more area on the substrate. Therefore, a high gain device requires a very large gain area. Each double in gain requires a double in area on the substrate. It also requires a more complicated op-amp, since it requires the op-amp output to drive a larger capacitor. In addition, a single-stage high-speed application requires high power consumption.